1. Field of the Invention
This invention relates to the art of receiving a Space Time Transmit Diversity (STTD) signal. In particular, this invention relates to frequency tracking of an STTD signal. The invention finds application in a closed-loop automatic frequency control in wireless user equipment. The invention is particularly well suited for use in Personal Digital Assistants, mobile communication devices, cellular phones, and wireless two-way e-mail communication devices (collectively referred to herein as “wireless devices”). The invention provides utility, however, in any device that receives an STTD signal.
2. Description of the Related Art
Space Time Transmit Diversity (STTD) reception is often mandatory for user equipment (UE), such as mobile communication devices, to operate in a standard fashion with various wireless communication radio network sub-systems (RNS), such as base stations. For example, in the 3rd Generation Partnership Project (3GPP) standard document No. 3GTS 25.211 V3.1.1 (1999-December), it is clearly indicated that STTD reception is mandatory for UE.
The concept of STTD transmission is known to those of skill in the art and involves the use of two transmit antennas at the RNS employing a space time block coding, such as the example illustrated in the block diagram of an STTD encoder of FIG. 1.
Although STTD transmission at an RNS is meant to be beneficial to reception at the UE, frequency tracking at the UE is complicated by STTD transmission.
Typically, in non-STTD systems, UE tracks an RNS pilot signal in order to control a local reference oscillator. The pilot signal is usually specifically designed in order to facilitate determining a frequency offset.
However, when the received signal from the base station is an STTD signal, detection of the frequency offset from the received signal is more difficult. FIG. 2 illustrates a typical pilot modulation pattern transmitted by STTD. The symbol A is a complex number with real and imaginary parts. In this particular case, the symbols from antenna 1 are always +A, while the symbols from antenna 2 are alternatively +A and −A with pattern shown. One problem with this pattern is that the two signal components can interfere with each other at the UE. Although not explicitly illustrated, one of skill in the art will appreciate that other patterns exist which present the same problem. Considering typical propagation conditions between RNS and UE, conventional methods for frequency tracking do not have sufficient performance to enable reliable frequency tracking. An alternative to frequency tracking is to rely on highly stable frequency reference source in the UE. However, this alternative is neither cost effective nor is it optimal from the receiver performance perspective.
There is a need for a method and apparatus for detecting frequency error between a frequency reference and a received STTD signal at UE. There is a further need for a method and apparatus that controls the frequency reference by tracking a received STTD signal at UE.